Vesicular arbuscular mycorrhizal (VAM) fungi enter into a beneficial symbiotic relationship with many plants. These fungi confer on the host plant resistance to plant pathogens and aid the plant in obtaining nutrients, particularly phosphorous, from the soil. Many of the crops capable of forming beneficial symbiotic relationships with VAM fungi are susceptible to common pathogenic fungi found in the rhizosphere.
A typical way of treating soil infected with pathogenic fungi or other pathogens is to treat the field in its entirety with a fungicide or fumigants. The most common techniques of fumigation, with agents such as methyl bromide, eliminate VAM fungi as well as the plant pathogens. Furthermore, serious health and safety concerns about these fumigants have recently arisen which may lead to a ban or severe restriction on continued use. Unfortunately, many of the fungicides which could replace the fumigants would be prohibitively expensive for application to an entire field.
Various methods of effecting beneficial infection by VAM fungi have been disclosed. For example, Nemec, Trop. Agric. (Trinidad) (1983) 60:97-101 describes the inoculation of citrus seedlings by dipping plant roots in an inoculum comprised of VAM fungi and a sticking agent, and the effect of various fumigants and fungicides on VAM fungi has been studied. See, e.g., Menge, Phytopathology (1982) 72:1125-1132; Nemec, Can. J. Bot. (1980) 58:522-526. In addition, Johnson, C. R. et al., J. Environ Hort (1985) 3:166-168 report the use of hydrophilic polymers as carriers for VAM inocula. In their experiments, four methods of inoculation were used--generally mixing the inoculum into the soil of potted plants; placing the inoculum directly beneath the root system of the cutting; dipping the root system into a slurry of "Terrasorb" which is a starch hydrolyzed polyacrylonitrile polymer using potassium hydroxide (manufactured by Industrial Services International, Bradenton, Fla.) at 1.5 g/100 mL water; and dipping the root system into a slurry of Viterra plant gel (comprised of potassium propenoate copolymers (manufactured by Nepera Chemical Company, Harriman, N.Y.) at 1 g/100 mL water. The particular polymers and dipping protocols used in the Johnson article are said to have resulted in lower inoculation levels than the alternative processes known in the art. In addition, the article by Beswetherick, J. T. et al., Trans. Br. Mycol. Soc. (1987) 89:603-605 describes the inoculation of roots with VAM by sandwiching the roots between squares of cellophane to which the fungal mycelium has been attached and references a number of other prior art techniques for inoculation.
Hwang, Plant Disease (1988) 72:448-452, describes the treatment of seeds with the fungicide metalaxyl followed by planting in soil inoculated with a VAM fungus. Groth et al., Plant Disease (1983) 67:1377-1378, describes the effect on plant growth of using soil treated with both metalaxyl and VAM fungi. Strider, Plant Disease Reporter (1977) 61:746-748, describes the use of benomyl in a root dip to help control Rhizoctonia root rot. German Democratic Republic Patent No. 128,396 (1977), is directed to method of controlling root diseases of cereals by coating the seeds with a nutrient substrate for antagonistic microflora and a fungicide. Leong, Ann. Rev. Phytopathol. (1986) 24:187-209, is a review of the role of siderophores in the biocontrol of plant pathogens. Melero-Vara et al., Plant Disease (1982) 66:132-135, is directed to a seed dressing containing metalaxyl to control downy mildew.
It is also known to coat the roots of seedlings during transplantation with a polymeric material to, for example, prevent desiccation. See, e.g., Hamilton et al., Tobacco International (1982) 184:88-91; Federal Republic of Germany Patent No. 1,945,110 (1970); Japanese Patent No. 57/083230 (1982); Japanese Patent No. 54/117734 (1979). In addition, U.S. Pat. Nos. 4,434,231 and 4,755,468, both to Jung, disclose crosslinking of polymer vehicles used to coat root systems by various means. Crosslinking, however, may suppress spore germination and infection.
Despite significant study of both the promotion of beneficial VAM fungi and the prevention of infection by plant pathogens, a continuing need exists for an improved, economical treatment of plants to promote growth and increase survival among transplanted seedlings. Several such methods are offered in the present invention.